1. Field of the Invention
The present invention relates generally to well casing fill apparatus and methods, and more particularly, to such apparatus and methods whereby a casing string is run in a well bore, filled with well bore fluid and cemented in the well bore.
2. Description of the Prior Art
In the construction of oil and gas wells, a well bore is drilled into one or more subterranean formations or zones containing oil and/or gas to be produced. The well bore is typically drilled utilizing a drilling rig which has a rotary table on its floor to rotate a pipe string during drilling and other operations.
During a well bore drilling operation, drilling fluid (also called drilling mud) is circulated through the well bore by pumping it down the drill string, through a drill bit connected thereto and upwardly back to the surface through the annulus between the walls of the well bore and the drill string. The circulation of the drilling fluid functions to lubricate the drill bit, remove cuttings from the well bore as they are produced and to exert hydrostatic pressure on pressurized fluid containing formations penetrated by the well bore whereby blow-outs are prevented.
In most instances, after the well bore is drilled, the drill string is removed and a string of casing is run into the well bore while maintaining sufficient drilling fluid in the well bore to prevent blow-outs. The term "casing string" is used herein to mean any string of pipe which is lowered into and cemented in a well bore including but not limited to surface casing, liners and the like.
During casing running operations, the casing string must be kept filled with fluid to prevent excessive fluid pressure differentials across the casing string and to prevent blow-outs. Heretofore, fluid has been added to the casing string at the surface after each additional casing joint is threadedly connected to the string and the string is lowered into the well bore. Also, well casing fill apparatus have heretofore been utilized at or near the bottom end of the casing string to allow well fluids in the well bore to enter the interior of the casing string while it is being run.
While prior casing fill apparatus have been used successfully, such apparatus have generally been complex and have not been completely reliable. That is, the fill valves associated with the apparatus have been susceptible to being accidently closed prior to completion of casing running operations without any method of reopening the valves. Further, prior casing fill apparatus cannot be used with certain types of single stage and multiple stage primary cementing equipment and/or require special procedures and apparatus for operating the fill valves. Thus, there is a continuing need for an improved casing string fill apparatus and methods of using the apparatus whereby the fill valves of the apparatus cannot be accidently closed prior to reaching total depth, the apparatus can be used with any type of single stage or multiple stage primary cementing equipment and the operation of the apparatus does not require special cementing plugs or changes in cementing practices. Further, there is a need for casing fill apparatus that can be made up in a casing string separately from the float equipment used or as an integral part of the float equipment.